Casings and pressed parts utilized for the extrusion of articles, particularly pipes, and manufacturing process of such casings and pressed parts

ABSTRACT

This invention relates to a capsule for pressings pressed by isostatic pressure and to these pressings used for extruding metallic objects, particularly tubes, of stainless steel, the outer and inner wall of the capsule consisting of thin-walled sheet metal, and at least the outer wall having substantially the same strength properties in the axial direction along its circumference and particularly consisting of a spiral-welded tube, and preferably at least on the front end of the capsule an insert being provided, which consists of one or more pieces of a ductile solid material or a ductile material pressed from powder. The invention further relates to a process for the production of such capsules and pressings and to a process for extruding tubes and to the tubes obtained according to this process.

This invention relates to a further development of the process for theproduction of stainless tubes described in German printed applicationDE-AS No. 24 19 014, corresponding to U.S. Pat. No. 4,143,208.

The German printed application DE-AS No. 24 19 014 and the correspondingU.S. patent relate to a process for the production of tubes of stainlesssteel having a uniform structure, uniform physical and chemicalproperties and good further processing properties, in which powder-formsteel of the type in question is introduced into metallic capsules; thecapsules are closed and compressed by a pressure acting on all sidesthereof and the pressing obtained is extruded into tubes, steel powderof predominantly spherical particles produced by sputtering melt in aninert gas atmosphere being used and the capsules used being thin-walledcapsules of a ductile metal having a maximum wall thicknesscorresponding to approximately 5% of the external diameter of thecapsule; the density of the steel powder introduced into the capsule isincreased to between about 60 and 70% of the theoretical density byvibration and/or ultrasound; the density of the steel powder isincreased to at least 80% and preferably to between 80% and 93% of thetheoretical density by isostatic cold pressing of the capsule under apressure of at least 1500 bars; the pressing is heated and subsequentlyhot-extruded, preferably at temperatures of at least about 1200° C., toform the required semi-finished product.

According to the German printed application DE-AS No. 24 19 014 it canbe of advantage to evacuate the metallic capsules filled with the steelpowder before they are closed and/or to fill them with a gas,particularly an inert gas, for example argon.

According to the German printed application DE-AS No. 24 19 014, it ispreferred to use metallic capsules of which the wall thickness amountsto less than 3% and more particularly to less than 1% of the externaldiameter of the capsule, metallic capsules having a wall thickness offrom about 0.1 to 5 mm and preferably from about 0.2 to 3 mm beingparticularly preferred.

According to said German printed application DE-AS No. 24 19 014, it isalso possible to produce composite tubes using thin-walled metalliccapsules which are separated by one or more concentric partitions intotwo or more compartments. The predominatly spherical powder particles ofthe various steel qualities are respectively introduced under vibrationinto one of these compartments, after which the partitions are removedand the capsules closed, followed by isostatic cold pressing andextrusion at elevated temperature. For extruding the pressings intotubes, glass is normally used as the lubricant. Since stringent demandsare imposed on the lubricant in the extrusion of, in particular,stainless steels at elevated temperatures, the pressing is required tohave a substantially flat end face so that the lubricant applied to theend face of the pressing in the form of a glass disc is effectivelyutilised.

It has now been found that the pressings frequently show deviations fromthe ideal shape and thus lead to difficulties and to rejects in theextrusion process.

The object of the present invention is to produce a capsule and apressing with which these difficulties of the known embodiments areavoided and with which the number of rejects and of defective productsis as low as possible.

According to the invention, this object is achieved in that at least theouter wall of the capsule has substantially the same strength propertiesin the axial direction over its entire circumference. According to theinvention, at least the outer wall of the capsule is preferably formedby a thin-walled, spiral-welded or extruded tube. Forming the outer wallof the capsule in this way affords the advantage that extruded products,particularly tubes, characterised by a considerably reduced number offaults and, hence, rejects are obtained.

It has further been found that extrusion is accompanied by thedevelopment of surface faults in the front part of the extruded producton account of the fact that the flow pattern in the transition betweenthe cover and the jacket is seriously disturbed through the disturbingeffect of welding. This causes significant losses in the yield of theend product.

Therefore it is a further object of the present invention to increasethe yield, i.e. to reduce the percentage of defective products afterextrusion.

According to the invention, this object is achieved in that the capsuleis constructed in such a way that a funnel-shaped insert is introducedat the end face of the capsule. This construction affords the advantagethat the flow properties during extrusion are improved, therebyincreasing the yield of stainless material.

According to the invention, the inserts preferably consist of anelectrically conductive metal, particularly soft iron or any otherinexpensive metal.

According to the invention, the inserts may be in the form of coverswhich close the capsule at its ends and may be tightly welded to theouter and inner wall of the capsule. Sheet metal inserts in the form ofcovers may with advantage also be provided between the inserts and theinterior of the capsules and may be tightly welded to the outer andinner walls.

According to the invention, it is possible in the case of capsules forthe production of pressings for extruding tubes to use funnel-shaped,centrally bored inserts for the front end face of the capsules, theangle 8' between the wall of the central bore for the inner wall of thecapsule and the conical outer surface of the funnel-shaped insertamounting to between about 40° and 60°, preferably to between about 40°and 50° and, more particularly, to about 45°.

According to the invention, it can be of advantage in the product oftubes to provide at least on the front end of the capsule a centrallybored annular insert which has a substantially flat end surface and ofwhich the boundary surface between the wall of the central bore and itslargest diameter has a substantially arcuate cross-sectional profile,the centre of the arcuate profile lying substantially in the vicinity ofthe intersecting line between the flat end face and the central bore.

Another significant improvement in the capsules and the pressings andextruded articles, particularly extruded tubes, produced from them, maybe obtained in combination with the above-described inserts constructedin accordance with the invention by ensuring that at least the outerwall of the capsule has substantially the same strength properties inthe axial direction over its entire circumference. According to theinvention, at least the outer wall of the capsule is preferably formedby a thin-walled, spiral-welded or extruded tube. Forming the outer wallof the capsule in this way affords the advantage that extruded products,particularly tubes are, characterised by a considerably reduced numberof faults and, hence, rejects are obtained. The insert members can alsobe pressed from powder material. For this purpose, for example powderobtained by water-atomizing soft iron or water-atomizing low-carbonsteel can be used, which powder is subjected to isostatic cold pressingto the desired shape of the aforementioned insert members andsubsequently to sintering. Cold isostatic pressing of the soft ironpowder can take place in a plastic mould, the pressure being preferrablyselected at least as high if not higher than the pressure for coldisostatic pressing which is applied to the afore-mentioned capsules. Bysubsequent hot sintering, a dense material can be obtained.

The present invention is applicable to capsules and pressings forextruding objects, particularly tubes, bars or similarly profiled,elongate, dense metallic objects, particularly of stainless steel orhighly alloyed nickel steels, particularly heat-resistant steels forheat exchangers, for example highly alloyed nickel steels containing 80%of nickel and 20% of chromium, powder of metal or metal alloys ormixtures thereof or mixtures of powders of metals and/or metal alloyswith ceramic powders being introduced into the capsule according to theinvention. The powder used is preferably spherical or predominantlyspherical powder having a mean particle diameter of preferably less thanapproximately 1 mm. According to the invention, it is preferred to usespherical powder which has been produced from the required startingmaterial, i.e. the required metal and/or metal alloy, by sputtering inan inert gas atmosphere, perferably an argon atomsphere. Powderparticles larger than 1 mm in diameter are preferably separated out, atleast to a predominant extent, because argon is in danger of beingincluded into powder particles larger than 1 mm in diameter. Aninclusion of argon such as this can occur during sputtering, for examplethrough turbulence. Any inclusion of argon would give rise duringextrusion to unfavourable properties of the extruded articles and wouldlead to inclusion lines.

According to the invention, the capsule used for producing the pressingsfor the tubes to be extruded is filled with the powder, the density ofthe powder introduced into the capsule being increased by vibration tobetween about 60 and 71% of the theoretical density and the frequency ofthe vibration preferably amounting to at least about 70 Hz andadvantageously to between 80 and 100 Hz. By vibration at a frequency offrom 80 to 100 Hz, it is possible to obtain a density of from about 68to 71% of the theoretical density.

After the powder has been introduced and compacted by vibration, thecapsule is closed, preferably after evacuation and/or filling with aninert gas. Thereafter the density of the powder is increased to at least80 to 93% of the theoretical density by isostatic cold pressing under apressure of at least 4000 bars, preferably under a pressure of from 4200to 6000 bars and, more particularly, under a pressure of from 4500 to5000 bars.

Capsules of generally thin sheet steel, preferably about 1 to 2 mm thicksheet steel and, more particularly, approximately 1.5 mm thick sheetsteel, have proved to be particularly advantageous. The material usedfor these capsules is preferably low-carbon soft steel, particularlysteel having a carbon content of less than 0.015% and, better still,less than 0.004% in order to prevent the powder from carburising duringheat and extrusion.

Under the effect of the pressure applied on all sides during coldisostatic pressing, the capsule is uniformly compressed both in thelongitudinal and in the radial direction and thus forms a pressing. Thispressing should have no irregularities because this would give rise todifficulties during extrusion, particularly during the extrusion oftubes.

In order to produce a pressing for extruding a tube, an annular capsuleis used, the outer wall of this annular capsule being formed by aspiral-welded tube section produced for example from an approximately1.5 mm thick sheet.

An inner wall, for example in the form of a longitudinally welded tubesection, is introduced into the interior of the outer wall, having asmaller diameter than but the same wall thickness as the outer wall. Anannular cover is then fixed between the outer wall and the inner wall atone end and the annular space between the two tubes is thus closed atone end. Spherical powder is then introduced into the annular space andcompacted to around 68% of the theoretical density by vibration at afrequency of, for example, 80 Hz. A vacuum is then applied and the otherend of the annular body is sealed off by a corresponding second cover.Thisis followed by cold isostatic pressing in a liquid, for examplewater, under a pressure of, for example, 4700 bars. Under the effect ofthe pressure applied on all sides, a pressing having a density of, forexample, 85% of the theoretical density is obtained.

In the capsule according to the invention, the spiral weld seam isrequired to be as smooth as possible with as little effect as possibleon the properties of the sheet steel. Accordingly, the weld seam ispreferably smoothed by rolling and/or grinding. The smoothing of theweld seam by rolling may be carried out immediately after welding.

In the case of capsules for the production of tubes, it can be ofadvantage to produce not only the outer wall, but also the inner wallfrom a tube which has substantially the same strength properties in theaxial direction along its circumference. In this case, the inner wallmay consist either of a spiral-welded tube or of an extruded tube. Theuse of an extruded or spiral-welded tube for the inner wall isparticularly advisable in the production of large tubes. In theproduction of smaller tubes, it is generally sufficient in accordancewith the invention for the outer wall of the capsule to be produced froma tube section which has substantially the same strength properties inthe axial direction along its circumference.

An embodiment of the invention is described by way of example in thefollowing with reference to the accompanying diagrammatic drawings,wherein:

FIG. 1 is an elevation of a capsule open at its upper end.

FIG. 2 is a longitudinal section through a modified embodiment of thecapsule.

FIGS. 3 and 4 are partial sections through other embodiments.

In FIG. 1, the capsule is generally denoted by the reference 1. Thecapsule has an outer wall 2 and inner wall 4. The outer wall 2 consistsof a spiral-welded tube section having a length L. The weld seam 5extends spirally over the circumference of the outer wall 2, the spiralhaving a helix angle α such that the spiral forms approximately onecomplete turn.

It has been found to be of advantage to arrange the weld seam 5 in sucha way that it forms one complete turn between the weld seam 16, which isused to weld the cover (not shown in FIG. 1) of the capsule firmly tothe outer wall 2, and the weld seam 26 by means of which the base of thecapsule is joined to the outer wall. The distance between the weld seams16 and 26 is denoted by the reference L' in FIG. 1. This length L' maybe regarded as the effective length of the capsule. It is advisable toselect the helix angle α of the spiral weld in such a way that

    tgα=L'/(n·π·D)

where D is the diameter of the capsule and n the number of turns whichthe spiral weld seam 5 is required to comprise. It has been found to beadvisable for n to have a value of 1. However, it may also be ofadvantage for n to have a value of 2,3,4 or to be equal to a largerwhole number.

In one practical example, the outer wall 2 and also the inner wall 4 ofthe capsule 1 consisted of 1.5 mm thick soft sheet steel having a carboncontent of less than 0.004%. The cover, which is not shown in FIG. 1,was welded in along the weld seam 16. To produce the pressing, powderwhich consisted predominantly of spherical particles having a meandiameter of less than 1 mm and which had been produced from the requiredstarting material, for example stainless steel, by sputtering in anargon atmosphere, was introduced into the capsule. After it had beenintroduced into the capsule, the powder was compacted to a density ofapproximately 68% of the theoretical density by vibration at a frequencyof 80 Hz. The capsule was then evacuated and closed by means of a cover.The cover was directly joined to the outer wall 2 of the capsule bywelding substantially along the line 16 in FIG. 1. In the example inquestion, the capsule had a length of 600 mm and an external diameter of150 mm. The internal diameter of the inner wall 4 was approximately 55mm. The inner wall 4 consisted of a longitudinally welded tube sectionwith a longitudinal weld seam 6. The powder was then compressed toaround 85% of the theoretical density by isostatic cold pressing under apressure of 4700 bars. The pressing thus obtained was extruded into atube as described in said German printed application DE-AS No. 24 19014.

In the embodiment illustrated in FIG. 2 plug-like, inserts 30 and 40 arearranged in the region of the cover 10 and the base 20, forming thefront and rear end face, respectively, of the capsule. The front insert30 is generally conical and comprises a central bore 32 for receivingthe inner wall 4 of the capsule. The conical surface 36 of the conicalor funnel-shaped insert 30 forms with the wall of the bore 32 an angle8' which is preferably in the range from about 40° to 60°,advantageously in the range from about 40° to about 50° and, moreparticularly, of the order of 45°. The insert 30 comprises asubstantially flat end face 34. However, it is bevelled or rounded offat its outer edge (at 35) and then comprises a cylindrical section 37which merges into the conical surface 36. The transition from theconical surface 36 to the wall of the central bore 32 is rounded off at39. The cover 10 in the form of a sheet-metal insert corresponds exactlyin its contour to the adjoining parts of the insert 30. Moreparticularly, the cover 10 comprises along its outer edge a cylindricalsection 17 which provides for firm contact between the cover 10 and theouter wall 2, the outer edge of this cylindrical section 17 being joinedto the outer wall 2 means of a weld seam 16. In its inner region, too,the cover 10 comprises a short, substantially cylindrical section 19which is in contact with the inner wall 4 of the capsule and which istightly welded to the inner wall 4 at 18 by means of a weld seam. Thecover 10 also comprises a rounding-off corresponding to the rounding-off39 of the insert 30.

Arranged at the rear end of the capsule 1 is an insert 40 in the form ofa substantially flat plate which comprises a central bore 42 and anoutwardly directed end face 44. This plate-like insert 40 is alsobevelled or rounded off at its edge (at 45) and comprises an outercylindrical section 47. The base 20 of the capsule corresponds in shapeto the shape of the insert 40 and also comprises an outer cylindricalsection 27 and an inner cylindrical section 29. The base 20 is tightlywelded to the outer wall 2 and the inner wall 4 by means of weld seams26 and 28, respectively. The inserts 30 and 40 preferably consist ofsoft iron or low carbon soft steel.

FIG. 3 shows a modified embodiment of the capsule in which a plug-likeinsert 130 provided at the front end of the capsule comprises asubstantially arcuate cross-sectional profile 136, a flat end face 134and a central bore 132. The centres of the arcuate cross-sectionalprofile 136 are situated on a circle substantially in the vicinty of theintersecting line between the flat end face 134 and the wall of the bore132, i.e. in the region of the front boundary line of the bore 132. Thiscircle is indicated by two crosses at 138 in FIG. 3. The substantiallyarcuate cross-sectional profile 136 affords the advantage that, duringextrusion of the pressing, the insert 130 consisting of soft iron or asimilar metal, together with the cover 110, the weld seams 116, 118 andthe adjacent parts of the outer wall 102 and the inner wall 104, formthe first part of the tube which is cut off or even drops offautomatically after extrusion if the connection to the following tubepreferably consisting of stainless steel and produced from the powderfilling of the capsule lacks sufficient strength. The effect of thesubstantially arcuate shape of the boundary line 136 of the insert 130is that the dividing line between the front part of the extruded tube,which accumulates as waste, and the actual tube consisting of highquality stainless steel is clearly defined and is in the form of aseparation surface extending substantially perpendicularly of thelongitudinal axis of the tube. The cover 110 also comprises asubstantially cylindrical section 117 which is welded at 116 to theouter wall 102 of the capsule, and a substantially cylindrical innersection 119 which is in contact with the inner wall 104 and which istightly welded to the inner wall at 118 by means of an encircling weldseam. The transition from the wall of the central bore 132 to thecircular cross-sectional profile 136 is rounded off at 139.

It can also be of advantage for the inserts 30 and 40 to be directlytightly welded to the outer wall 2 and the inner wall 4. In this case,the cover 10 and the base 20 may be omitted. Similarly, the insert shownin FIG. 3 may be directly tightly welded to the outer wall 102 and tothe inner wall 104.

In cases where sheet metal inserts are used as cover and base, it may beof advantage to join the inserts 30, 40, 130 to them by spot welding. Inmany cases, however, it is also sufficient to fix the inserts 30, 40 and130 through the flanged ends 15, 25 and 115 of the outer wall 2 and 102.

During extrusion, the insert at the front end of the capsule leads to atype of tunnel effect providing it is made of ductile material, forexample ductile iron, soft iron, low-alloyed carbon steel or cast iron.The pressure required in the container of the extrusion press forextruding the pressing is reduced where the front insert consists ofductile material which can be made to flow more easily than the powderfilling of the pressing. Once the flow process taking place duringextrusion has started, it also affects the powder filling, even when theyield point of the powder filling is higher than the yield point of theductile material of the insert. Accordingly, a type of tunnel effect isobtained.

Moreover, the pressing was also completely straight and, after inductiveheating to 1200° C., could be directly extruded to form the requiredstainless and seamless tube without any need for further machiningoperations. The front section of the tube consisting of low-alloyedcarbon steel was cut off. None of the stainless steel was cut off. Byvirtue of the fact that the insert is conical, a substantially vertical(relative to the tube axis) parting line between the extruded insert andthe stainless steel was obtained in the extruded tube. That part of thetube which consisted of stainless material had a fault-free surface. Inthis way, the loss of material was reduced to a minimum.

In order to obtain good separation between the front section of theextruded tube, which consists of a low-alloyed carbon steel, and thedesired seamless tube of stainless steel, a layer of glass can accordingto the invention be applied to the surface of the cover 10 or 110 facingthe powder filling 8, 108. It can be of advantage for this purpose toheat the cover 10 or 110 and to sprinkle glass powder onto their outersurfaces, the temperature of the insert member being selected so thatthe glass powder becomes soft and adheres. By such an intermediate layerof glass, the separation between the low-alloyed carbon steel and thestainless steel is made very much easier when the extruded tube isobtained so that the two steel grades are obtained completely separatelyfrom each other and without mixing.

Similarly, also the surface of the base 20, which is adjacent to thepowder filling 8 on the bottom of the capsule can be provided with alayer of glass facilitating a separation of the stainless steel materialand the low-alloyed carbon steel.

The curved or rounded-off insert members 30, 40, and 130 can also bepressed from powder material. For this purpose, for example powderobtained by water-atomizing soft iron or water-atomizing low-carbonsteel can be used, which powder is subjected to isostatic cold pressingto the desired shape of the aforementioned insert members andsubsequently to sintering. Isostatic cold pressing of the soft ironpowder can take place in a plastic mould, the pressure being preferrablyselected at least as high if not higher than the pressure for isostaticcold pressing which is applied to the aforementioned capsules. Bysubsequent hot sintering, a dense material can be obtained.Alternatively or additionally, in this case also a sealing can beobtained at the front end faces 34, 134, and 334 respectively 44 as wellas on the circumferential surfaces by applying an outer layer of glassthereon.

The embodiment according to FIG. 4 corresponds extensively to theembodiment according to FIG. 3. Only the upper insert members have amodified shape. The front insert 330' comprises two rings 380 and 381held together by several spot weldings 382. Instead of two rings 380,381, of course three or more rings can also be provided, whereby theouter contour of such rings constitutes an approximation to the idealcontour of the front insert member which is determined by the arcuatecross-section 136 in FIG. 3.

A substantial difference consists in that no sheet metal inserts areprovided between the insert 330' and the powder filling 308, but theouter wall 301 at 316 and the inner wall 304 at 318 are directly tightlywelded to the insert or its upper ring, respectively. The ring 381 isbevelled or rounded off at 335 similar to the rounding-off 35 of theinsert 30 and the rounding-off 135 of the insert 130. In the embodimentaccording to FIG. 3, the bottom insert, which is not shown, consists ofan annular plate which is also directly tightly welded to the outer walland the inner wall so that a bottom metal sheet can be omitted.

All particulars and features disclosed in the documents, particularlythe spatial configuration disclosed, are claimed as being essential tothe invention where they are new either individually or in combinationin relation to the prior art.

We claim:
 1. A capsule having an annular transverse cross section forisostatically producing a pressing for use in the extrusion of densemetallic tubes, said capsule comprising a tubular thin-walled sheetmetal container to be filled with a powder and having plug-like insertsfor closing each of its opposite ends, wherein at least the insert atthe front end of the capsule is formed of ductile material, has agenerally annular shape with a central bore, is provided with asubstantially flat end face at the front end of the capsule, and has anexternal diameter which decreases rearwardly from a point of maximumexternal diameter to a wall defining said central bore, and wherein atleast the outer wall of the thin-walled sheet metal container hassubstantially the same strength properties in the axial direction alongits entire circumference.
 2. A capsule according to claim 1, wherein theaxial strength properties of at least said outer wall are madesubstantially the same along its entire circumference by said wallhaving been formed from a strip of sheet metal which is wound into acylindrical form having a spiral seam and spiral-welded along said seam.3. A capsule according to claim 2, wherein said plug-like inserts arecovered by sheet metal inserts at a side facing the interior of thecapsule.
 4. A capsule as claimed in claim 3, characterized in that thesheet metal inserts (10, 20) are joined to the outer wall (2) andoptionally to the inner wall (4) of the capsule (1) by welds (16, 18,26, 28).
 5. A capsule as claimed in claims 1 or 2, characterised in thatthe inserts (30, 40) are welded to the outer wall (2) and optionally tothe inner wall (4) of the capsule.
 6. Capsules as claimed in claim 2,characterised in that the pitch (α) of the spiral formed by the weldseam (5) in relation to the length (L) of the capsule (1) is such thatthe weld seam (5) forms substantially one, two or more complete turns.7. Capsules as claimed in one of claims 2 or 6, characterized in thatthe spiral weld seam is smoothed by at least one of rolling andgrinding.
 8. A capsule according to claim 1, wherein the axial strengthproperties of at least said outer wall are made substantially the samealong its entire circumference by said wall having been formed byextrusion.
 9. A capsule according to claim 1 or 2 or 8, wherein saiddecreasing external diameter forms a conical outer surface.
 10. Acapsule according to claim 9, wherein an angle of between 40 and 60degrees is formed between said conical outer surface and said centralbore.
 11. A capsule according to claim 1 or 2 or 8, wherein saiddecreasing external diameter forms an outer surface having arcuatecorss-sectional profiles.
 12. A capsule according to claim 11, whereinthe centers of the arcuate cross-sectional profiles of said outersurface lie on a circle formed substantially in the vicinity of anintersecting line between said flat end face and said central bore. 13.A capsule as claimed in claim 2 or 9 or 11, characterized in that theinserts (30, 40) consist of an electrically conductive metal, preferablysoft iron.
 14. A pressing for the extrusion of tubes, comprising atubular capsule formed of a tubular thin-walled sheet metal containerhaving inner and outer peripheral walls, preferably low-carbonsheet-steel having a carbon content of less than 0.015 percent andpreferably less than 0.004 percent, wherein at least the outer wall ofthe thin-walled sheet metal container has substantially the samestrength properties in the axial direction along its entirecircumference, the capsule being filled with powder from the groupconsisting of metal or metal alloys or mixtures thereof, or mixturesthereof with ceramic powders, which preferably consists of spherical orpredominantly spherical inert gas atomized particles, and the density ofthe powder introduced into the capsule having been increased to at least80 to 93 percent of the theoretical density by isostatic cold pressingof said capsule, and wherein said capsule is closed at each of oppositeends by a plug-like insert, wherein at least the insert at the front endof the capsule is formed of ductile material, has a generally annularshape with a central bore, is provided with a substantially flat endface at the front end of the capsule, and has an external diameter whichdecreases rearwardly from a point of maximum external diameter to a walldefining said central bore.
 15. A process for producing pressings asclaimed in claim 14, wherein at least the insert provided at the frontend is made of a ductile metal, for example soft iron, low-carbon steelor cast iron, of which the yield point in a container of an extrusionpress is considerably below the yield point of the powder filling of thepressing so that the extrusion process begins under the pressurerequired for the ductile material of the insert and is transferred bytunnel effect to the powder filling.
 16. A pressing according to claim14, wherein said decreasing external diameter forms a conical outersurface.
 17. A pressing according to claim 16, wherein an angle ofbetween 40 and 60 degrees is formed between said conical outer surfaceand said central bore.
 18. A pressing according to claim 14, whereinsaid decreasing external diameter forms an outer surface having arcuatecross-sectional profiles.
 19. A pressing according to claim 18, whereinthe centers of the arcuate cross-sectional profiles of said outersurface lie on a circle formed substantially in the vicinity of anintersecting line between said flat end face and said central bore. 20.A process for producing a dense metallic tube, said process comprisingthe steps of providing a tubular, thin-walled, sheet metal container,said container having an outer and inner peripheral wall, wherein atleast the outer wall of the thin-walled sheet metal container hassubstantially the same strength properties in the axial direction alongits entire circumference, providing plug-like inserts for closing theopposite ends of said container, wherein at least the insert at thefront end of the capsule is formed of ductile material, has a generallyannular shape with a central bore, is provided with a substantially flatend face at the front end of the capsule, and has an external diameterwhich decreases rearwardly from a point of maximum external diameter toa wall defining said central bore, filling said capsule with a powderselected from the group consisting of metal or metal alloys, or mixturesthereof with ceramic powders, increasing the density of said powder byvibration to between 60 and 70 percent of the theoretical density,sealing said inserts to the opposite ends of said container and furtherincreasing the density of said powder to at least 80 to 93 percent ofthe theoretical density by isostatic cold pressing to produce apressing, and extruding said pressing in an extrusion means to form saiddense metallic tube.
 21. Process according to claim 20, characterized inthat for obtaining good lubrication when extruding, glass is used forlubrication, which is placed in form of a glass disc on the front end(34, 134, 334) of the pressing in the container or receiver of theextrusion press, is supplied by means of a bevelled front edge (35, 135,335) of the front insert (30, 130, 330') and a very precise adaptationof the substantially exactly cylindric external diameter of the pressingto the substantially cylindric internal diameter of the container orreceiver of the extrusion press, during the entire extrusion operation,substantially uniformly distributed in the circumferential directionbetween tool and extruded object.